Specifically, in 4.0.7 the A component is centered relative to the window and the X component. In 4.1-pr1 it is left aligned.

A minor difference I noticed is that in Ext 4.1-pr1 autoSize no longer does anything, but I don't think that's relevant to my issue because autoSize was only needed in 4.0.7 to get the vbox to appear at all, whereas in 4.1 it appears without it, it's just that the alignment seems wrong.